Non-aqueous electrolyte secondary battery

ABSTRACT

In a non-aqueous electrolyte secondary battery having: an electrode plate assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode; a non-aqueous electrolyte including a lithium salt and a non-aqueous solvent; and a gas absorbing element that absorbs gas produced in the secondary battery, wetting of the gas absorbing element with the non-aqueous solvent is controlled.

BACKGROUND OF THE INVENTION

Non-aqueous electrolyte secondary batteries of high energy density havewidely been used with the cordless and portable trend of AV equipment,personal computers, and other electronic apparatuses. Especially lithiumsecondary batteries are most advanced in practical use. The non-aqueouselectrolyte secondary battery has a high electromotive force ofapproximately 4 V and a high energy density exceeding 350 Wh/L.

Available examples of the non-aqueous electrolyte secondary batteryinclude cylindrical batteries and rectangular batteries. In thecylindrical batteries, a positive electrode plate and a negativeelectrode plate are wound via a separator and are accommodated, togetherwith a non-aqueous electrolyte in a cylindrical case. In the rectangularbatteries, an electrode plate assembly wound in a flat-sided shape isaccommodated in a thin rectangular case.

Polymer secondary batteries have recently been in practical use. In thepolymer secondary batteries, a stack of electrode plates obtained byinterposing a polymer electrolyte between adjoining electrode plates iswrapped with a laminate sheet of a resin film and a metal foil. A gelelectrolyte including a non-aqueous liquid electrolyte kept in a polymermatrix is applied for the polymer electrolyte.

The non-aqueous electrolyte secondary battery has high electromotiveforce, so that the non-aqueous solvent in the electrolyte is readilydecomposed. Decomposition of the non-aqueous solvent causes generationof gas, for example, CH₄, C₂H₄, C₂H₆, CO, CO₂, and H₂, inside thebattery. Methane and carbon dioxide are primary components of the gas.

The production of the gas is accelerated when the battery is kept athigh temperatures for a long time period, is used at high temperatures,or is overcharged. The produced gas raises the internal pressure of thebattery and may deform or damage the case. The produced gas alsoaccelerates deterioration of the battery characteristics. Especially inthe case of the polymer secondary battery, the bulge due to the producedgas causes the polymer electrolyte to be peeled off the electrode plate,and deteriorates the characteristics to the fatal level.

By taking into account generation of the gas due to decomposition of thenon-aqueous solvent, the battery is provided with a safety valve that isactivated at a preset pressure or a safety mechanism that detects thepressure and cuts off the electric current. An increase in internalpressure of the battery, however, causes frequent activation of thesafety valve, which leads to release of the components of theelectrolyte as well as the gas and thereby badly affects electronicapparatuses. The high working pressure of the safety valve, on the otherhand, leads to easy deformation of a battery case.

In the non-aqueous electrolyte secondary battery containing thenon-aqueous solvent, decomposition of the non-aqueous solvent isinevitable. Means for solving the above problems have thus been highlydemanded. The following techniques have been proposed to control anincrease in internal pressure of the battery due to the produced gas:

(i) Japanese laid-open patent publication No. 6-267593 discloses abattery including a substance capable of absorbing the produced gas or asubstance reacting with the gas. This also discloses a positiveelectrode and a negative electrode with such a substance applied on thesurface thereof, as well as a separator with such a substance containedtherein.

(ii) Japanese laid-open patent publication No. 11-191400 discloses amulti-layered battery having gas blocking property and rigidity. Thisbattery has a plastic inner housing, and includes a moisture absorbentor a gas absorbent, for example, any of silica gel, zeolite, activecarbon, metal salts like stearates, hydrosulfites, and hydrogenabsorbing alloys.

(iii) Japanese laid-open patent publication No. 9-180760 shows amechanism of making the gas produced inside the battery, for example,hydrogen, methane, ethane, and carbon monoxide, electrochemically reactwith oxides or Ketchen Black added to the electrode plates.

(iv) Japanese laid-open patent publication No. 11-224670 describes thatcarbon materials, such as active carbon or carbon black have a capacityof absorbing carbon dioxide, carbon monoxide, nitrogen, and argon.

(v) Japanese laid-open patent publication No. 11-54154 discloses abattery including an alkali earth metal oxide (for example, SrO, CaO,BaO, or MgO) for fixation of carbon dioxide. Any of these oxides may beused in a powdery form or as a molded article.

(vi) Japanese laid-open patent publication No. 2000-90971 discloses apositive electrode including active carbon as a gas absorbent and alithium-containing transition metal oxide.

As described above, diverse efforts have been made to control theincreasing internal pressure of the battery due to the produced gas andprevent the resulting decrease in reliability in the non-aqueouselectrolyte secondary battery.

The prior art technique, however, does not attain long-term stablecontrol of the increasing internal pressure of the battery. This isbecause the conventional gas absorbent is readily wetted (excessivelywetted) with the non-aqueous solvent of the non-aqueous electrolyte. Theexcessive wetting of the conventional gas absorbent with the non-aqueoussolvent extremely lowers the capacity of gas absorption. Functionalgroups, such as carbonyl group, carboxyl group, aldehydes group, andhydroxide group are present on the surface of active carbon and carbonblack and are expected to accelerate the wetting. Active carbon, forexample, is manufactured by firing natural fiber material or syntheticfiber material at relatively low temperatures of 350 to 650° C., whichsuppress crystallization of carbon, and reforming (activating) the firedfiber material with an acid, an alkali, steam, or zinc chloride. Thereforming process increases the surface area of the carbon material,while forming a large number of the functional groups.

Any of the conventional gas absorbents has poor capacity of absorbingmethane and carbon dioxide. Since methane and carbon dioxide are theprimary components of the gas produced in the battery, only the materialhaving sufficient capacity of absorbing these gases can effectivelyprevent an increase in internal pressure of the battery.

The battery is generally manufactured in the air or nitrogen, so thatthe gas absorbent is sealed in the battery after absorption of the airor nitrogen to its saturated level. The gas absorbent that has alreadyabsorbed a large quantity of the air or nitrogen can not sufficientlyabsorb the gas produced in the battery. From the viewpoint of attainingthe enhanced productivity and the less scattering and loss of organicsolvents, however, it is extremely difficult to manufacture the batteryunder reduced pressure, with a view to preventing the gas absorbent fromabsorbing the air or nitrogen.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is thus to control wetting of a gasabsorbent with a non-aqueous solvent and ensure long-term, stable actionof the gas absorbent, thus enhancing the reliability of a non-aqueouselectrolyte secondary battery. The object of the present invention isalso to use a gas absorbent, which is not readily wetted with anon-aqueous solvent and has a capacity of selectively absorbing methaneor carbon dioxide, thus enhancing the reliability of a non-aqueouselectrolyte secondary battery.

The present invention is accordingly directed to a non-aqueouselectrolyte secondary battery, which comprises: an electrode plateassembly including a positive electrode, a negative electrode, and aseparator interposed between the positive electrode and the negativeelectrode; a non-aqueous electrolyte including a lithium salt and anon-aqueous solvent; and a gas absorbing element capable of absorbinggas produced in the secondary battery. The gas absorbing elementincludes a gas absorbent and a lyophobic agent against the non-aqueoussolvent.

The gas absorbent is capable of absorbing, for example, at least one gasselected from the group consisting of methane, ethane, ethylene, carbondioxide, and hydrogen. It is preferable that the gas absorbent comprisesat least one selected from the group consisting of carbon materials,zeolite, metals, metal oxides, metal nitrides, and intermetalliccompounds.

The lyophobic agent may comprise a polymer material as discussed later.It is preferable that dibutyl phthalate absorption of the lyophobicagent is not greater than 150 ml/100 g.

In one preferable application, the gas absorbing element comprises apowdery mixture including the gas absorbent and the lyophobic agent. Acontent of the lyophobic agent in the powdery mixture is preferably 2 to30 parts by weight relative to 100 parts by weight of the gas absorbent.It is preferable that dibutyl phthalate absorption of the powderymixture is not greater than 150 ml/100 g. The gas absorbing element maybe either one of a molded article and a sintered article of the powderymixture.

In another preferable application, a difference between surface freeenergy of the gas absorbing element and surface free energy of thenon-aqueous electrolyte is 5 to 50 mN/m at 20° C. For example, it ispreferable that a difference between surface free energy of the moldedarticle or the sintered article and surface free energy of thenon-aqueous electrolyte is 5 to 50 mN/m at 20° C.

In still another preferable application, the gas absorbing element has aporous layer that prevents the gas absorbent from being wettedexcessively by the non-aqueous electrolyte, and the porous layercomprises the lyophobic agent. It is preferable that the porous layercovers over the gas absorbent.

A difference between surface free energy of the porous layer and surfacefree energy of the non-aqueous electrolyte is preferably 5 to 50 mN/m at20° C. It is desirable that at least either one of the gas absorbent andthe porous layer is a molded article or a sintered article.

In another preferable application, the gas absorbing element is acoating film that is formed on a surface of a constituent of the batteryand includes the gas absorbent and the lyophobic agent. In still anotherpreferable application, the gas absorbing element has a coating filmthat is formed on a surface of a constituent of the battery and includesthe gas absorbent, and a porous layer that includes the lyophobic agentand covers over the coating film.

In one preferable embodiment, the electrode plate assembly is wound, andthe gas absorbing element is serving as a core of the wound electrodeplate assembly.

In one preferable example, when the electrode plate assembly isaccommodated in a rectangular case, the gas absorbing element has aplate-like form and is serving as the core. It is preferable that anadditional gas absorbing element comprising a coating film including thegas absorbent and the lyophobic agent is provided on the inner face ofthe flat rectangular case.

In another preferable example, when the electrode plate assembly isaccommodated in a cylindrical case, the gas absorbing element has abar-like form and is serving as the core. It is preferable that anadditional gas absorbing element comprising a coating film including thegas absorbent and the lyophobic agent is provided on the inner face ofthe cylindrical case.

In another preferable application, the gas absorbing element is fixed toa sealing plate, which seals an opening of a casing for accommodatingthe electrode plate assembly therein.

The present invention is also directed to a non-aqueous electrolytesecondary battery, which comprises: an electrode plate assemblyincluding a positive electrode, a negative electrode, and a separatorinterposed between the positive electrode and the negative electrode; anon-aqueous electrolyte including a lithium salt and a non-aqueoussolvent; and a gas absorbing element capable of absorbing gas producedin the secondary battery, wherein the gas absorbing element comprises agas absorbent capable of selectively absorbing at least one selectedfrom the group consisting of carbon dioxide and methane. The gasabsorbent may be included, for example, in at least either one of thepositive electrode and the negative electrode.

It is preferable that the gas absorbent has a specific surface area of300 to 1500 m²/g, and that a ratio of number of oxygen atoms to numberof carbon atoms in the gas absorbent, that is, O/C ratio, is not greaterthan 0.1.

It is also preferable that a content of the gas absorbent in the batteryis not less than 0.2 g per 1000 mAh of cell capacity.

The present invention is further directed to a method of manufacturing anon-aqueous electrolyte secondary battery. The method includes the stepsof: (1) utilizing powdery carbon material to prepare a gas absorbentcapable of selectively absorbing at least one selected from the groupconsisting of carbon dioxide and methane; (2) preparing an electrodematerial mixture including the gas absorbent and an electrode activematerial and applying the electrode material mixture on a collector toobtain an electrode; and (3) assembling the electrode, a separator, anda non-aqueous electrolyte to a non-aqueous electrolyte secondarybattery.

In one preferable application, the step (1) heats the powdery carbonmaterial in a benzene atmosphere at 600 to 1000° C., so as to makebenzene chemically adsorbed on the powdery carbon material. It ispreferable that the benzene atmosphere is a gaseous mixture atmospherecontaining nitrogen and 1 to 10% by volume. of benzene. It is alsopreferable that the gaseous mixture has a pressure of 1×10⁵ to 2×10⁵ Pa.

In another preferable application, the step (1) heats the powdery carbonmaterial in an inert atmosphere at 600 to 1300° C. for 10 to 120minutes.

It is preferable that the powdery carbon material is at least oneselected from the group consisting of carbon black and active carbon. Itis also preferable that the powdery carbon material has a specificsurface area of 50 to 1500 m²/g.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a partly omitted, oblique view illustrating a thin polymerbattery in accordance with the present invention;

FIG. 2 is a partly decomposed view illustrating a rectangular battery inaccordance with the present invention;

FIG. 3 is a partly omitted, oblique view illustrating a cylindricalbattery in accordance with the present invention;

FIG. 4 is a partly omitted, front view illustrating a gas absorbingelement, which comprises a powdery mixture including a gas absorbent anda lyophobic agent;

FIG. 5 is a oblique view illustrating one gas absorbing element, whichis a molded article of the powdery mixture including the gas absorbentand the lyophobic agent;

FIG. 6 is a oblique view illustrating another gas absorbing element,which is a molded article of the powdery mixture including the gasabsorbent and the lyophobic agent;

FIG. 7 is a partly omitted, front view illustrating a gas absorbingelement including a gas absorbent, and a powdery lyophobic agent thatprevents the gas absorbent from being wetted excessively by anon-aqueous electrolyte;

FIG. 8 is a partly omitted, front view illustrating a gas absorbingelement including a gas absorbent, a porous layer that prevents the gasabsorbent from being wetted excessively by a non-aqueous electrolyte,and a powdery lyophobic agent;

FIG. 9 is a partly omitted, front view illustrating a gas absorbingelement including a molded article of a gas absorbent, and a porouslayer that prevents the molded article from being wetted excessively bya non-aqueous electrolyte;

FIG. 10 is a front view illustrating a gas absorbing element fixed to anelectrode lead;

FIG. 11 is a sectional view illustrating a gas absorbing elementaccommodated in a space of a sealing plate;

FIG. 12 is a sectional view illustrating a gas absorbing element fixedto a holder of a sealing plate;

FIG. 13 is a sectional view illustrating a gas absorbing element that isformed at a predetermined site inside a battery and has a coating filmincluding a gas absorbent, and a porous layer covering over the coatingfilm;

FIG. 14 is a graph showing the cycle life of the battery plotted againstthe DBP absorption of a powdery mixture in a charge-discharge cycle testof batteries Lb_(n) in Example 4;

FIG. 15 shows a manufacturing process of a gas absorbent capable ofselectively absorbing carbon dioxide;

FIG. 16 shows a process of forming pores that selectively take in carbondioxide;

FIG. 17 is a graph showing the weight decreasing rate of a gas absorbent(X) that has absorbed carbon dioxide to its saturated level and a carbonmaterial (Y) plotted against the temperature in differential thermalanalysis; and

FIG. 18 is a graph showing the discharge capacity plotted against thenumber of charge-discharge cycles with regard to batteries of Examplesand Comparative Examples.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

In this embodiment, a gas absorbing element comprises a powdery mixtureincluding a gas absorbent and a lyophobic agent against a non-aqueoussolvent.

It is preferred that the gas absorbent selectively absorbs decomposedgas components of the non-aqueous solvent. The decomposed gas includesmethane, ethane, ethylene, carbon dioxide, and hydrogen. Carbon dioxideand methane are especially abundant.

The shape and the particle size of the gas absorbent may be setarbitrarily. For example, the gas absorbent having a mean particlediameter of 10 to 500 μm is available. Typical examples of such gasabsorbent include carbon materials, zeolite, metals, metal oxides, metalnitrides, and intermetallic compounds. Especially preferable are carbonmaterials.

The carbon material may be active carbon or carbon black. The carbonmaterial may be reformed to a gas absorbent that preferentially absorbsmethane and carbon dioxide by heat treatment at 600 to 1300° C. or byheat treatment in a flow of benzene at 600 t 1300° C. as discussedlater.

Zeolite may be molecular sieves or the like. Typical examples of themetal oxide include aluminum oxide and silica. Typical examples of themetal and the intermetallic compound include palladium, nickel, LaNi₅,MgNi, and TiFe.

The lyophobic agent prevents the gas absorbent from being wetted with anon-aqueous solvent and thereby enables the gas absorbent to have thelong-term stable capacity of gas absorption. The affinity of thelyophobic agent to the non-aqueous solvent is evaluated by dibutylphthalate (hereinafter referred to as DBP) absorption. The DBPabsorption represents a quantity of DBP absorbed by 100 g of thelyophobic agent. The DBP absorption is obtained by soaking the powderylyophobic agent in DBP, removing excess DBP, measuring the weight of theDBP-containing lyophobic agent, and calculating a difference between theobserved weight and the original weight of the powdery lyophobic agent.The less DBP absorption shows the higher lyophobic property. In order tosufficiently prevent the gas absorbent from being wetted with thenon-aqueous solvent, the DBP absorption of the lyophobic agent ispreferably not greater than 150 ml/100 g.

Typical examples of the lyophobic agent include polyethylene,polypropylene, polytetrafluoroethylene, polyvinylidene fluoride,polyacrylonitrile, polyimide, copolymer of ethylene and propylene,copolymer of ethylene and vinyl acetate, copolymer oftetrafluoroethylene and hexafluoropropylene, copolymer of styrene andbutadiene (hereinafter referred to as SBR), and terpolymer of ethylene,propylene, and vinyl acetate. Any of these polymers may be used alone orin combination. Especially preferable are polyethylene, polypropylene,polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile,polyimide, copolymer of tetrafluoroethylene and hexafluoropropylene, andSBR. Any of these lyophobic agents is available in a powdery form. Thelyophobic agent has a mean particle diameter of, for example, 0.5 to 10μm.

In order to prevent the non-aqueous solvent from reaching the gasabsorbent while allowing the gas produced inside the battery to reachthe gas absorbent, a gas passage should be formed among the particles ofthe lyophobic agent or among the particles of the lyophobic agent andthe particles of the gas absorbent. One preferable state is mutualdispersion of the particles of the gas absorbent and the particles ofthe lyophobic agent, which is attained by mixing the powdery gasabsorbent with the powdery lyophobic agent. Another preferable state isthat primary particles or secondary particles of the gas absorbent arecoated with fine powder of the lyophobic agent.

A V-shaped blender or a high-speed jet blender is more desirably used,than a mixing module for kneading the gas absorbent and the lyophobicagent, to prepare the gas absorbing element having adequate lyophobicproperty and gas permeability. Such blenders enable the surface of thegas absorbent particles to be coated with the fine particles of thelyophobic agent, without damaging the shape of the particles. Anotheravailable process is soaking the gas absorbent in an emulsion ordispersion of the lyophobic agent and drying the soaked gas absorbent tomake the fine particles of the lyophobic agent deposit on the surface ofthe gas absorbent particles.

Still another applicable process is soaking the gas absorbent in asolution of the lyophobic agent and drying the soaked gas absorbent tocoat the surface of the gas absorbent particles with the film of thelyophobic agent. Another applicable process is soaking the gas absorbentin a solution of a starting monomer or oligomer of the lyophobic agentand polymerizes the monomer or oligomer to coat the surface of the gasabsorbent particles with the film of the lyophobic agent. In the case ofmixing a polyvinylidene fluoride-containing solution with the gasabsorbent, coexistence of polyethylene, polypropylene, SBR or the likein the solution is desirable.

For the adequate lyophobic property and gas permeability of the gasabsorbing element, the powdery mixture of the gas absorbent and thelyophobic agent preferably has DBP absorption of not greater than 150ml/100 g. The DBP absorption may be varied according to the mixing stateof the powdery mixture.

The powdery mixture of the gas absorbent and the lyophobic agent ispreferably molded. Sintering the molded article of the powdery mixturegives the gas absorbing element having excellent lyophobic property andsufficiently high strength.

A binding agent may be added to the powdery mixture of the gas absorbentand the lyophobic agent according to the requirements. Typical examplesof the binding agent include polyolefin, carboxymethyl cellulose, andpolyvinylidene fluoride.

It is preferable that a difference between surface free energy of themolded or sintered gas absorbing element and surface free energy of anon-aqueous electrolyte used for the battery is 5 to 50 mN/m at 20° C.For example, the non-aqueous electrolyte is obtained by dissolving LiPF₆in a 1:1 (volume ratio) solvent mixture of ethylene carbonate anddiethyl carbonate to a concentration of 1 mol/liter.

FIG. 1 is a partly omitted, oblique view illustrating a thin polymerbattery in accordance with the present invention, which includes a gasabsorbing element. In this polymer battery, an electrode plate assembly113 including a positive electrode plate 101 interposed between a pairof negative electrode plates 102 via separators 103 is accommodated in alaminate film case 106 comprising a resin and aluminum. A positiveelectrode lead 104 is connected to the positive electrode plate 101,whereas a negative electrode lead 105 is connected to the negativeelectrode plate 102. Each lead is drawn out of the opening of the case106 via a hot melt resin 107. The outer ends of the respective leadsform a positive electrode outer terminal 110 and a negative electrodeouter terminal 111. A resin film 108 functioning as a safety valve isprovided between the positive electrode lead 104 and the negativeelectrode lead 105 at the opening of the case 106. There is a relativelyweak joint between the resin film 108 and the inner face of the case.The joint surface is peeled off to release the gas to the outside withan increase in internal pressure of the battery. In the structure ofFIG. 1, a gas absorbing element 109 formed in a rectangular solid islocated in a space between the electrode plate assembly 113 and theopening of the case 106. This structure ensures effective use of thespace inside the battery.

FIG. 2 is a partly decomposed view illustrating a rectangular batteryincluding a gas absorbing element. In this battery, an electrode plateassembly 213 including a positive electrode plate 201 and a negativeelectrode plate 202 laid one upon the other via a separator 203 androlled to a flat-sided shape is accommodated in a thin case 206. Apositive electrode lead 204 is connected to the positive electrode plate201, whereas a negative electrode lead 205 is connected to the negativeelectrode plate 202. The outer ends of the respective leads form apositive electrode outer terminal 210 and a negative electrode outerterminal 211. A plate-like gas absorbing element 209 is serving as acore of the electrode plate assembly 213. In the rectangular battery,the opening of the case 206 is sealed with a sealing plate 212 having asafety valve 208. The safety valve 208 is, for example, a clad platehaving a crack. The crack opens to release the gas to the outside whenthe internal pressure of the battery reaches or exceeds a preset value.

FIG. 3 is a partly omitted, oblique view illustrating a cylindricalbattery including a gas absorbing element. In this battery, an electrodeplate assembly 313 including a positive electrode plate and a negativeelectrode plate laid one upon the other via a separator and wound in acylindrical shape is accommodated in a cylindrical case 306. A bar-likegas absorbing element 309 is serving as a core of the electrode plateassembly 313. The opening of the case 306 is sealed with a sealing plate312 having a safety valve and a positive electrode outer terminal 310.The safety valve is, for example, a rubber valve 308. The rubber valve308 closes an aperture formed in the sealing plate to connect the insidewith the outside of the battery. When the internal pressure of thebattery reaches or exceeds a preset value, the rubber valve 308 isdeformed to release the gas to the outside. In the illustration of FIGS.1 through 3, a non-aqueous electrolyte is omitted. In the illustrationof FIG. 3, the positive electrode lead is omitted.

In the batteries having the wound electrode plate assembly as shown inFIGS. 2 and 3, application of the gas absorbing element for the core ofthe electrode plate assembly ensures effective use of the space insidethe battery and is advantageous in the following points. Application ofthe gas absorbing element for the core facilitates the winding processof the electrode plates and prevents deformation of the electrode plateassembly, thus being advantageous in manufacture and improving thecharacteristics of the battery. This also readily attains the uniformdistance between the positive electrode plate and the negative electrodeplate in the electrode plate assembly, thus significantly enhancing thecycle property. Under application of a large shock on the battery, thegas absorbing element located on the center of the electrode plateassembly is broken to form a large number of short circuits. Theelectric current is thus not concentrated on one short circuit andthereby enhances the safety.

Any suitable gas absorbing element other than those discussed above maybe applied for the thin polymer battery, the rectangular battery, andthe cylindrical battery.

FIG. 4 shows a gas absorbing element 409, which includes a powderymixture of a gas absorbent 414 and a lyophobic agent 415 and a container416 for accommodating the powdery mixture therein. A gas passage 417should be formed in the container 416. The case 416 may be made of anyarbitrary material, but a material having resistance against thenon-aqueous electrolyte, for example, a fluorocarbon resin likepolytetrafluoroethylene is preferable. The case 416 in the shape of arectangular solid shown in FIG. 4 may be replaced with a case or a bagof porous material. This gas absorbing element is especially preferablefor large-sized non-aqueous electrolyte secondary batteries used inelectric vehicles or the like.

FIG. 5 shows a gas absorbing element 509 obtained by molding a powderymixture of a gas absorbent and a lyophobic agent in a disc shape. Thisgas absorbing element is suitably located on the end face of theelectrode plate assembly in a cylindrical shape.

FIG. 6 shows a gas absorbing element 609 obtained by molding a powderymixture of a gas absorbent and a lyophobic agent in a rectangular solid.This gas absorbing element is suitably received, for example, in a thinpolymer battery, since the gas absorbing element of this shape isreadily inserted together with the electrode plate assembly into thecase and is advantageous in manufacture. The case of the thin polymerbattery has an inlet, through which the non-aqueous electrolyte isinjected. The gas absorbing element 609 may thus be inserted through theinlet, prior to sealing. The inlet is cut and sealed after theinjection.

A film-like gas absorbing element may advantageously be formed byspraying and drying an emulsion or a dispersion containing the powderymixture on the constituent of the battery. One exemplified process isspraying and drying an emulsion or a dispersion containing the powderymixture to form a film-like gas absorbing element integrated with theinner face of the case. The cylindrical battery has high energy densityand produces a large amount of gas. Combination of the bar-like gasabsorbing element with the film-like gas absorbing element formed on theinner face of the case is thus especially effective for the cylindricalbattery.

Embodiment 2

In this embodiment, the lyophobic agent prevents the gas absorbent frombeing wetted excessively by the non-aqueous electrolyte.

A gas absorbing element 709 shown in FIG. 7 is similar to the gasabsorbing element 409 shown in FIG. 4, except that a gas absorbent 714and a lyophobic agent 715 are not mixed with each other but the powderylyophobic agent 715 is arranged to surround the powdery gas absorbent714. The lyophobic property of this gas absorbing element is evaluatedby DBP absorption of the lyophobic agent 715.

In a gas absorbing element 809 shown in FIG. 8, only a gas absorbent 814is contained in a container 816, and a gas passage 817 formed in thecontainer 816 is covered with a porous layer 818 comprising thelyophobic agent. The gas absorbing element 809 also has a space thatcommunicates with the gas passage 817 and accommodates a lyophobic agent815 therein.

In a gas absorbing element 909 shown in FIG. 9, a molded article 919 ofthe gas absorbent is surrounded by a porous layer 918 comprising thelyophobic agent. When the gas absorbing element has the porous layercomprising the lyophobic agent, the lyophobic property of the gasabsorbing element is evaluated by a difference between surface freeenergy of the porous layer and surface free energy of the non-aqueouselectrolyte.

The gas absorbing element composed of the molded article of the gasabsorbent and the porous layer as shown in FIG. 9 is produced, forexample, according to any of the following processes:

(1) The process of spraying and firing the powdery lyophobic agent ontothe molded article of the gas absorbent;

(2) The process of mixing a binding agent with the powdery lyophobicagent, applying the resulting mixture onto the molded article of the gasabsorbent, and firing the molded article when necessary; and

(3) The process of mixing an aperture-forming material with thelyophobic agent, applying the resulting mixture onto the molded articleof the gas absorbent, and removing the aperture-forming material byfiring or extraction.

The molded article of the gas absorbent preferably has a plate-likeshape or a bar-like shape to be used as the core of the electrode plateassembly as shown in FIGS. 1 through 3.

Embodiment 3

This embodiment regards the accommodated structures of the gas absorbingelement different from those of FIGS. 1 through 3. In the followingstructures, the gas absorbing element is able to be received at apredetermined position simultaneously with insertion of the electrodeplate assembly into the case or with attachment of the sealing plate tothe case. This advantageously simplifies the battery manufacturingprocess.

FIG. 10 shows one accommodated structure of the gas absorbing elementsuitable for the battery of FIG. 1 or FIG. 2. In this structure, a gasabsorbing element 1009 is fixed on a film 1020 attached to a positiveelectrode lead 1004 and/or a negative electrode lead 1005 with anadhesive or the like. Fixation of the gas absorbing element 1009 may beattained by the adhesive. It is preferable that the film 1020 isinsulative and is electrochemically inactive. This construction enablesthe gas absorbing element 1009 to be received in the case simultaneouslywith insertion of an electrode plate assembly 1013 into the case.

FIG. 11 shows a gas absorbing element 1109, which is accommodated in areceiving portion 1121 attached to the inner face of a sealing plate1112. A gas passage 1117 is formed in the receiving portion 1121 and iscovered with a porous layer 1118 comprising the lyophobic agent. Thisconstruction enables the gas absorbing element 1109 to be received inthe case simultaneously with attachment of the sealing plate 1112 to thecase. Alternatively only a gas absorbent may be accommodated in thereceiving portion 1121.

FIG. 12 shows a gas absorbing element 1209 that is fixed to a holder1222 provided on a sealing plate 1212. The holder 1222 may be providedon a lead 1204 of an electrode plate or on the inner wall of a case,instead of the sealing plate 1212.

In the structure of FIG. 13, a coating film 1323 that is formed at apredetermined site 1300 inside the battery and contains a gas absorbent1314 is covered with a porous layer 1318 comprising the lyophobic agent.The site 1300 at which the coating film 1323 is formed may be the innerwall of a case, the inner face of a sealing plate, or part of a lead.The coating film 1323 may be formed, for example, by applying a mixtureof the gas absorbent and a binding agent onto the predetermined site.The porous layer 1318 may be formed, for example, by applying a mixtureof the powdery lyophobic agent and the binding agent onto the coatingfilm 1323. Available examples the binding agent include polyolefin,carboxymethyl cellulose, and polyvinylidene fluoride.

Embodiment 4

This embodiment regards a gas absorbent that selectively absorbs carbondioxide and methane (hereinafter referred to as the selective gasabsorbent). The selective gas absorbent has the lyophobic property tothe non-aqueous solvent and accordingly does not require any independentlyophobic agent. The selective gas absorbent does not absorb a largequantity of the air, and still has the capacity of absorbing carbondioxide and methane even after absorption of the air to its saturatedlevel. The selective gas absorbent is produced, for example, frompowdery carbon. The conventional gas absorbent, such as active carbon orcarbon black, on the other hand, has various sized pores and diversefunctional groups and indiscriminately absorbs a diversity of gases. Thegas absorbing capacity of the conventional gas absorbent left in the airis accordingly saturated soon.

The following describes some methods of manufacturing the selective gasabsorbent.

(i) Method 1

This method makes benzene chemically adsorbed on a powdery carbonmaterial to manufacture the selective gas absorbent. An apparatus shownin FIG. 15 is suitable for this method.

In the apparatus of FIG. 15, an airtight container 1502 is packed with apowdery carbon material 1501. The container 1502 communicates with a gassupply conduit 1503 and a gas exhaust conduit 1504. A gaseous mixturecontaining benzene and nitrogen is flown through the gas supply conduit1503 and is fed into the airtight container 1502. The gaseous mixturecontaining benzene and nitrogen is produced in a benzene reservoir 1505.A nitrogen gas inlet 1507 is located below the fluid level of benzene1506 in the benzene reservoir 1505. When nitrogen gas is fed through thenitrogen gas inlet 1507 into the benzene 1506, the gaseous mixturecontaining benzene and nitrogen is produced. The exhaust gas from theairtight container 1502 is fed to a nitrogen gas supply conduit 1508 bymeans of a pump 1509 and recycled. The powdery carbon material 1501 isheated in a furnace 1510, together with the gaseous mixture containingbenzene and nitrogen fed into the airtight container 1502. When heatingstarts, the benzene contained in the gaseous mixture is chemicallyadsorbed on the surface of the powdery carbon material to reform thepowdery carbon material.

FIG. 16 shows chemical adsorption of benzene on the surface of thepowdery carbon material. A large number of relatively large pores 1602to allow passage of benzene molecules 1606 are present on the surface ofpowdery carbon material 1601. Multiple benzene molecules 1606 arechemically adsorbed on the wall surface of the pore 1602 to form a giantplanar molecule 1603. Another benzene molecule 1606 is furtherchemically adsorbed on the planar molecule 1603. The opening of the pore1602 is gradually narrowed through such iterative chemical adsorption togive a resulting pore 1604 that does not allow passage of the benzenemolecule. The final aperture of the pore is desirable for insertion oflinear carbon dioxide molecules 1608.

The preferable heating temperature of the powdery carbon material rangesfrom 600 to 1000° C. The heating at temperature of lower than 600° C.results in poor chemical adsorption of benzene. The heating temperatureof higher than 1000° C., on the other hand, causes benzene toexcessively cover the surface of the powdery carbon material and doesnot give the gas absorbent having desired pores. The heating timedepends upon the composition of the gaseous mixture containing benzeneand nitrogen, but is preferably in a range of 1 to 5 hours.

The pressure of the gaseous mixture containing benzene and nitrogeninside the airtight container 1502 in the course of heating ispreferably kept in a range of 1×10⁵ to 2×10⁵ Pa. The preferred contentof benzene in the gaseous mixture is 1 to 10% by volume. The desirabletemperature range of the benzene 1506 kept in the benzene reservoir 1505is 30 to 50° C.

Typical examples of the powdery carbon material include active carbon,carbon black, natural graphite, artificial graphite, anthracite coal,and sintered matters of natural fibers, resins, fats, and oils. They maybe used alone or in combination. Among them, carbon black and activecarbon are especially preferable. The powdery carbon material preferablyhas a mean particle diameter of 0.05 to 50 μm.

(ii) Method 2

This method heats the powdery carbon material in an inert atmosphere tomanufacture the selective gas absorbent. The process comprises heatingthe powdery carbon material in an inert atmosphere at 600 to 1300° C.for 10 through 120 minutes. A nitrogen or argon atmosphere is preferablefor the inert atmosphere. Preferable examples of the powdery carbonmaterial are given in Method 1. Method 2 is suitable for mass productionand is industrially preferable.

Heating the powdery carbon material at temperatures of 600 to 1300° C.induces adequate detachment of the functional groups present on thesurface of the powdery carbon material and contraction of the powderycarbon. In this process, pores of specific size that allow selectiveinsertion of carbon dioxide and methane are formed on the surface of thepowdery carbon.

The heating temperature of lower than 600° C. leaves most of thefunctional groups present on the surface of the powdery carbon material,so that the resulting gas absorbent does not have sufficient selectivityfor carbon dioxide and methane. The heating temperature of higher than1300° C., on the other hand, causes graphitization and excessivecontraction of the powdery carbon, thus significantly decreasing thesurface area.

The following describes the preferred properties of the selective gasabsorbent.

The selective gas absorbent is less wettable with the non-aqueoussolvent than the powdery carbon material. The selective gas absorbentcontained in the battery and exposed to the non-aqueous electrolytestill keeps sufficient gas absorption capacity. For example, the DBPabsorption of the selective gas absorbent, which has been subjected toabsorption of the air to its saturated level in a 1×10⁵ Pa atmosphere ofthe air at 25° C., is not greater than 150 ml/100 g. The DBP absorptionof the powdery carbon material, on the other hand, exceeds 300 ml/100 g.

The selective gas absorbent preferably has a specific surface area of300 to 1500 m²/g. The ratio of the number of oxygen atoms to the numberof carbon atoms, the O/C ratio, in the selective gas absorbent ispreferably not greater than 0.1.

It is desirable that the selective gas absorbent has the capacity ofabsorbing carbon dioxide, methane, or gaseous mixture of carbon dioxideand methane by a volume of at least 10 times that of the air, after2-hour degasification at 400° C. This property of the gas absorbent isevaluated, for example, with the air, carbon dioxide, methane, and thegaseous mixture of carbon dioxide and methane having a pressure of10×10⁵ Pa. The selective gas absorbent located in an airtight containerincluding a sample gas, such as carbon dioxide, methane, or the airvaries the pressure of the sample gas. The quantity of the sample gasabsorbed by the selective gas absorbent is calculated from the pressurevariation.

It is desirable that the selective gas absorbent has the capacity ofabsorbing at least 100 ml/g of carbon dioxide, methane, or gaseousmixture of carbon dioxide and methane in a 1×10⁵ Pa atmosphere of carbondioxide, methane, or the gaseous mixture of carbon dioxide and methaneat 25° C., after absorption of the air to its saturated level in a 1×10⁵Pa atmosphere of the air at 25° C.

It is also desirable that the weight decreasing rate at 100° C. of theselective gas absorbent, which has been subjected to 2-hourdegasification at 400° C. and has absorbed carbon dioxide to itssaturate level in a 1.013×10⁵ Pa atmosphere of carbon dioxide at 25° C.,in a flow of Ar by differential thermal analysis is not greater than 10%of the weight decreasing rate at 500° C.

A curve X in the graph of FIG. 17 represents the weight decreasing rateplotted against the temperature when the selective gas absorbentmanufactured according to Method 1 has been subjected to 2-hourdegasification at 400° C., has absorbed carbon dioxide to its saturatedlevel, and is heated in a flow of Ar. A curve Y represents the weightdecreasing rate plotted against the temperature when the powdery carbonmaterial is heated in a flow of Ar.

While the weight decreasing rate of the selective gas absorbent at 200°C. is less than 10%, the weight decreasing rate of the powdery carbonmaterial exceeds 20%. There is no such a difference when the air is usedinstead of carbon dioxide. This proves that carbon dioxide is stronglyadsorbed on the selective gas absorbent, compared with the powderycarbon material. The weight decreasing rate at 100° C. of the selectivegas absorbent, which has absorbed carbon dioxide to its saturated level,in a flow of Ar by differential thermal analysis is approximately 4% ofthe weight decreasing rate at 500° C. This also proves strong adsorptionof carbon dioxide on the selective gas absorbent.

The selective gas absorbent may be molded or sintered and located at anyarbitrary position in the battery. The selective gas absorbent may bemixed with a thickening agent and applied in any arbitrary site insidethe battery. As discussed above in Embodiments 1 through 3, theselective gas absorbent may also be combined with the lyophobic agent.The selective gas absorbent may be included in at least either one of apositive electrode material mixture and a negative electrode materialmixture.

The positive electrode material mixture includes, for example, apositive electrode active material, a conducting material, and a bindingagent. These materials are kneaded with the selective gas absorbent anda disperse medium to yield paste or slurry positive electrode materialmixture containing the selective gas absorbent. In the case where theselective gas absorbent is electrically conductive, the selective gasabsorbent may be used instead of the conducting material. This positiveelectrode material mixture is applied on a current collector, dried,rolled, and cut to a predetermined shape. This process gives a positiveelectrode.

The negative electrode material mixture includes, for example, anegative electrode material and a binding agent. These materials arekneaded with the selective gas absorbent and a disperse medium to yieldpaste or slurry negative electrode material mixture containing theselective gas absorbent. This negative electrode material mixture isapplied on a current collector, dried, rolled, and cut to apredetermined shape. This process gives a negative electrode.

Any of metal foils, metal films, metal sheets, meshes, lath plates,punched metals may be applicable for the current collector. In onepreferable example, the positive electrode collector is composed ofaluminum, while the negative electrode collector is composed of copper.

Li-containing transition metal oxides are applicable for the positiveelectrode active material. Typical examples of the Li-containingtransition metal oxide include LiCoO₂, LiMnO₂, Li₂MnO₄, LiMn₂O₄ andLiNiO₂. These may be used alone or in combination.

Metal lithium, artificial graphite, natural graphite, and amorphouscarbon are applicable for the negative electrode material. These may beused alone or in combination.

Carbon materials, such as graphite powder, active carbon, carbon black,and carbon fibers are applicable for the conducting material. These maybe used alone or in combination.

Fluorocarbon resins are applicable for the binding agent. Typicalexamples of the fluorocarbon resin include polyvinylidene fluoride,polyhexafluoropropylene, polytetrafluoroethylene, and vinylidenefluoride-hexafluoropropylene copolymer. These may be used alone or incombination.

Water and organic solvents are applicable for the disperse medium.N-methyl-2-pyrrolidone is a desirable disperse medium, since itfacilitates kneading of the electrode material mixture and acceleratesdrying of the electrode material mixture.

A reinforcing agent, such as polymer fillers, and a viscosity controlagent other than the above materials may be included in the electrodematerial mixture.

The non-aqueous electrolyte preferably includes a lithium salt and anon-aqueous solvent that dissolves the lithium salt. Typical examples ofthe lithium salt include LiBF₄ and LiPF₆. These may be used alone or incombination. Typical examples of the non-aqueous solvent includeethylene carbonate, propylene carbonate, ethyl methyl carbonate,dimethyl carbonate, and diethyl carbonate. These may be used alone or incombination.

Generation of a decomposed gas at a rate of 15 ml or a greater volumeper 1000 mAh of the battery capacity inside the non-aqueous electrolytesecondary battery damages the safety of the battery. For absorption of15 ml or the greater volume of the decomposed gas, at least 0.2 g of thegas absorbent is required.

The present invention is discussed more concretely with examples.

EXAMPLE 1

A thin polymer battery as shown in FIG. 1 was manufactured.

(i) Production of Gas Absorbing Element

Active carbon obtained by activating carbon black (acetylene black) withKOH was used as a gas absorbent. The DBP absorption of the gas absorbent(carbon black-type active carbon) was 250 ml/100 g.Polytetrafluoroethylene (hereinafter referred to as PTFE) having a meanparticle diameter of 1 μm was applied for the lyophobic agent. The DBPabsorption of the PTFE powder was 20 ml/100 g.

100 parts by weight of the gas absorbent was mixed with 25 parts byweight of the lyophobic agent by means of a gas jet blender to yield apowdery mixture. The DBP absorption of the resulting powdery mixture was30 ml/100

The powdery mixture was molded under a pressure of 150 kgf/cm² to arectangular solid of 5 mm in length, 15 mm in width, and 2 mm inthickness, and the molded article was sintered in a flow of nitrogen at300° C. for 30 minutes. This gave a gas absorbing element of arectangular solid. The surface free energy of the rectangular solid gasabsorbing element was 15 mN/m at 20° C.

The non-aqueous electrolyte used in this Example 1 and subsequentExamples 2 through 8 and Comparative Examples 1 through 4 was preparedby dissolving LiPF₆ in a 1:1 (volume ratio) solvent mixture of ethylenecarbonate and diethyl carbonate at a concentration of 1 mol/liter. Thesurface free energy of the non-aqueous electrolyte was 40 mN/m at 20° C.

(ii) Production of Thin Polymer Battery

(a) Preparation of Positive Electrode

100 parts by weight of LiCoO₂ as the positive electrode active material,5 parts by weight of carbon black as the conducting material, 8 parts byweight of a copolymer including 90% by weight of vinylidene fluorideunit and 10% by weight of hexafluoropropylene unit (hereinafter referredto as PVDF-HFP), and an adequate quantity of N-methyl-2-pyrrolidone werekneaded to yield a positive electrode material mixture. PVDF-HFP wasdissolved in N-methyl-2-pyrrolidone. The positive electrode materialmixture was then applied on one face of an aluminum foil currentcollector having a thickness of 20 μm, rolled, dried, and cut to apredetermined size. This gave a positive electrode plate having athickness of 125 μm. A positive electrode lead was connected to thepositive electrode plate.

(b) Preparation of Negative Electrode

100 parts by weight of artificial graphite as the negative electrodematerial, 14 parts by weight of PVDF-HFP, and an adequate quantity ofN-methyl-2-pyrrolidone were kneaded to yield a negative electrodematerial mixture. PVDF-HFP was dissolved in N-methyl-2-pyrrolidone. Thenegative electrode material mixture was then applied on both faces of acopper foil current collector having a thickness of 10 μm, rolled,dried, and cut to a predetermined size. This gave a negative electrodeplate having a thickness of 265 μm. A negative electrode lead wasconnected to the negative electrode plate.

(c) Assembly of Battery

One positive electrode plate was interposed between two negativeelectrode plates via separator layers to assemble a stack of electrodeplates. A mixture of PVDF-HFP and N-methyl-2-pyrrolidone was used forthe separator layers. The stack of electrode plates was inserted into abag-shaped case of a laminate sheet including a resin film and analuminum foil.

The opening of the case was subsequently sealed, except an inlet for theelectolyte, via the two leads and a film (safety valve) comprising acopolymer of ethylene and acrylic acid. The safety valve was set to openwhen the internal pressure of the battery was higher than theatmospheric pressure by 1.5 kgf/cm². The rectangular solid gas absorbingelement was inserted through the inlet, and the non-aqueous electrolytewas then injected through the inlet. The inlet was finally sealed tocomplete a thin polymer battery (La) as shown in FIG. 1. The resultingthin polymer battery (La) had the thickness of 3.6 mm, the width of 63mm, the length of 70 mm, and the capacity of 1150 mAh.

EXAMPLE 2

A rectangular battery as shown in FIG. 2 was manufactured.

(i) Production of Gas Absorbing Element

The same powdery mixture prepared in Example 1 was molded under apressure of 150 kgf/cm² to a plate-like shape of 15 mm in length, 47 mmin width, and 0.3 mm in thickness, and the molded article was sinteredin a flow of nitrogen at 300° C. for 30 minutes. This gave a plate-likegas absorbing element. The surface free energy of the plate-like gasabsorbing element was 17 mN/m at 20° C.

(ii) Production of Rectangular Battery

(a) Preparation of Positive Electrode

100 parts by weight of LiCoO₂ as the positive electrode active material,3 parts by weight of carbon black as the conducting material, 4 parts byweight of polyvinylidene fluoride, and an adequate quantity ofN-methyl-2-pyrrolidone were kneaded to yield a positive electrodematerial mixture. Polyvinylidene fluoride was dissolved inN-methyl-2-pyrrolidone. The positive electrode material mixture was thenapplied on an aluminum foil current collector having a thickness of 20μm, rolled, dried, and cut to a predetermined size. This gave a positiveelectrode plate having a thickness of 140 μm. A positive electrode leadwas connected to the positive electrode plate.

(b) Preparation of Negative Electrode

100 parts by weight of artificial graphite as the negative electrodematerial, 8 parts by weight of polyvinylidene fluoride, and an adequatequantity of N-methyl-2-pyrrolidone were kneaded to yield a negativeelectrode material mixture. Polyvinylidene fluoride was dissolved inN-methyl-2-pyrrolidone. The negative electrode material mixture was thenapplied on a copper foil current collector having a thickness of 10 μm,rolled, dried, and cut to a predetermined size. This gave a negativeelectrode plate having a thickness of 150 μm. A negative electrode leadwas connected to the negative electrode plate.

(c) Assembly of Battery

The positive electrode plate and the negative electrode plate were laidone upon the other via a polypropylene porous separator, and wound to aflat-sided shape around the plate-like gas absorbing element as the coreto assemble an electrode plate assembly. Application of the plate-likegas absorbing element for the core facilitated the winding process ofthe electrode plate assemly and did not require removal of the core fromthe electrode plate assembly. Namely this construction significantlysimplifies the manufacturing process, compared with the conventionalrectangular batteries.

The electrode plate assembly was inserted into a thin rectangular case,and the non-aqueous electrolyte was then injected into the case.Application of the gas absorbing element for the core effectivelyprevented deformation of the electrode plate assembly during itsinsertion, and remarkably reduced the percent of defective. The openingof the case was sealed with a sealing plate having the ends of the twoleads and a safety valve. This completed a rectangular battery (Ma) asshown in FIG. 2. The safety valve was set to open when the internalpressure of the battery was higher than the atmospheric pressure by 3kgf/cm². The resulting rectangular battery (Ma) had the thickness of 6.3mm, the width of 34 mm, the length of 50 mm, and the capacity of 850mAh.

EXAMPLE 3

A cylindrical battery as shown in FIG. 3 was manufactured.

(i) Production of Gas Absorbing Element

The same powdery mixture prepared in Example 1 was molded under apressure of 150 kgf/cm² to a semicircle-ended bar-like shape of 57 mm inlength and 1.2 mm in diameter of the semicircle, and the molded articlewas sintered in a flow of nitrogen at 300° C. for 30 minutes. This gavea bar-like gas absorbing element. The surface free energy of thebar-like gas absorbing element was 17 mN/m at 20° C.

An aqueous emulsion was prepared including 100 parts by weight of thegas absorbent (carbon black-type active carbon) prepared in Example 1and 5 parts by weight of PTFE powder having a mean particle diameter of1 μm. The resulting emulsion was sprayed onto the inner face of acylindrical case and dried to form an additional gas absorbing elementof a coating film having a thickness of 0.5 mm. The surface free energyof the coating film was 20 mN/m at 20° C.

(ii) Production of Cylindrical Battery

A positive electrode plate and a negative electrode plate were preparedin the same manner as Example 2. The positive electrode plate and thenegative electrode plate were laid one upon the other via apolypropylene porous separator, with the ends of the laminate clampedbetween the flat parts of two bar-like gas absorbing elements, and thelaminate was wound around the two gas absorbing elements as the core ina spiral form. This gave a cylindrical electrode plate assembly.

The electrode plate assembly was inserted into the cylindrical caseprovided with the additional gas absorbing element of the coating film,and the non-aqueous electrolyte was then injected into the case. Theopening of the case was sealed with a sealing plate having a safetyvalve and functioning as a positive electrode outer terminal. Thiscompleted a cylindrical battery (Na) as shown in FIG. 3. The safetyvalve was set to open when the internal pressure of the battery washigher than 12 kgf/cm². The resulting cylindrical battery (Na) had thediameter of 18.3 mm, the height of 65 mm, and the capacity of 1800 mAh.

As in the case of the rectangular battery, application of the gasabsorbing element for the core in the cylindrical battery simplified themanufacturing process and effectively prevented deformation of theelectrode plate assembly.

COMPARATIVE EXAMPLE 1

100 parts by weight of the gas absorbent (carbon black-type activecarbon) prepared in Example 1 was mixed with 5 parts by weight ofsaccharose and an adequate quantity of water. The mixture was moldedunder a pressure of 150 kgf/cm² to a rectangular solid of 5 mm inlength, 15 mm in width, and 2 mm in thickness, and the molded articlewas sintered in a flow of nitrogen at 300° C. for 30 minutes. This gavea gas absorbing element of a rectangular solid without the lyophobicagent. The surface free energy of the rectangular solid gas absorbingelement without the lyophobic agent was 37 mN/m at 20° C. A thin polymerbattery (Lr) was then manufactured in the same manner as Example 1,except that the rectangular solid gas absorbing element without thelyophobic agent was used.

COMPARATIVE EXAMPLE 2

100 parts by weight of the gas absorbent (carbon black-type activecarbon) prepared in Example 1 was mixed with 5 parts by weight ofsaccharose and an adequate quantity of water. The mixture was moldedunder a pressure of 150 kgf/cm² to a plate-like shape of 15 mm inlength, 47 mm in width, and 0.3 mm in thickness, and the molded articlewas sintered in a flow of nitrogen at 300° C. for 30 minutes. This gavea plate-like gas absorbing element without the lyophobic agent. Thesurface free energy of the plate-like gas absorbing element without thelyophobic agent was 38 mN/m at 20° C. A rectangular battery (Mr) wasthen manufactured in the same manner as Example 2, except that theplate-like gas absorbing element without the lyophobic agent was used.

COMPARATIVE EXAMPLE 3

100 parts by weight of the gas absorbent (carbon black-type activecarbon) prepared in Example 1 was mixed with 5 parts by weight ofsaccharose and an adequate quantity of water. The mixture was moldedunder a pressure of 150 kgf/cm² to a semicircle-ended bar-like shape of1.2 mm in diameter of the semicircle and 57 mm in length, and the moldedarticle was sintered in a flow of nitrogen at 300° C. for 30 minutes.This gave a bar-like gas absorbing element without the lyophobic agent.The surface free energy of the bar-like gas absorbing element withoutthe lyophobic agent was 38 mN/m at 20° C.

A coating film of 0.5 mm in thickness was formed on the inner face of acylindrical case in the same manner as Example 3, except that theaqueous emulsion did not contain PTFE powder but contained the carbonblack-type active carbon prepared in Example 1. The surface free energyof the coating film was 36 mN/m at 20° C. A cylindrical battery (Nr) wasmanufactured in the same manner as Example 3, except that the bar-likegas absorbing element without the lyophobic agent and the cylindricalcase with the coating film not including the lyophobic agent were used.

Evaluation of Batteries 1

The batteries of Examples 1 through 3 and Comparative Examples 1 through3 were evaluated as discussed below. Prior to the test, each battery wasinitially charged to 4.20 V with an electric current of 0.1 C (10-hourrate).

Each battery was then subjected to a charge-discharge cycle test at 45°C. The test repeated a cycle, which discharged each battery to 3.0 Vwith an electric current of 1C, charged the battery to 4.25 V with anelectric current of 0.7 C, and further charged the battery at a fixedvoltage to attain an electric current level of 0.05 C. A cycle life wasmeasured as the number of cycles until the capacity became less than 60%of the initial capacity in the first cycle or until the safety valveworked. The less sufficient absorption of the gas generated in thebattery by the gas absorbing element results in the smaller number ofcycles. The results of the evaluation test are shown in Table 1.

TABLE 1 Cycle Life Examples Batteries Type of Battery (times) Ex. 1 LaThin polymer battery 330 Ex. 2 Ma Rectangular battery 380 Ex. 3 NaCylindrical battery 375 Com. Ex. 1 Lr Thin polymer battery 50 Com. Ex. 2Mr Rectangular battery 60 Com. Ex. 3 Nr Cylindrical battery 70

As shown in Table 1, any of the batteries Lr, Mr, and Nr in ComparativeExamples, where the gas absorbing element did not include the lyophobicagent, had the life of shorter than 100 cycles. Any of the batteries La,Ma, and Na in Examples, on the other hand, had the life of longer than300 cycles.

After the test, each battery was decomposed for the purpose ofobservation of the gas absorbing element. In the batteries ofComparative Examples that did not reach 100 cycles, the gas absorbingelement was significantly wetted with the non-aqueous electrolyte. Inthe batteries of Examples that exceeded 300 cycles, on the other hand,the gas absorbing element still possessed the ability of repelling thenon-aqueous electrolyte. This proves efficient gas absorption.

As clearly understood, the lyophobic agent enables the gas absorbent toefficiently absorb-the gas and thereby stabilizes the batterycharacteristics.

EXAMPLE 4

Powdery mixtures having different DBP absorptions were produced in thesame manner as Example 1, except that the mixing ratio of the PTFEpowder to the gas absorbent (carbon black-type active carbon) was variedin a range of 2 to 30 parts by weight relative to 100 parts by weight ofthe gas absorbent. The DBP absorption of the powdery mixture containing2 parts by weight of the PTFE powder relative to 100 parts by weight ofthe gas absorbent was 180 ml/100 g. The DBP absorption of the powderymixture containing 30 parts by weight of the PTFE powder relative to 100parts by weight of the gas absorbent was 30 ml/100 g. The DBP absorptionof the other powdery mixtures was in the range of 30 to 180 ml/100 g.

Each of the powdery mixtures was molded under a pressure of 150 kgf/cm²to a plate-like shape of 15 mm in length, 47 mm in width, and 0.3 mm inthickness, and was sintered in a flow of nitrogen at 300° C. for 30minutes. This gave a plate-like gas absorbing element like Example 2.Rectangular batteries (Mb_(n)) were then manufactured in the same manneras Example 2, except that the respective gas absorbing elements wereused.

Evaluation of Batteries 2

The batteries Mb_(n) of Example 4 were subjected to the charge-dischargecycle test according to the procedure discussed in Evaluation ofBatteries 1. The graph of FIG. 14 shows the cycle life of the batteryplotted against the DBP absorption of the powdery mixture. As shown inFIG. 14, the batteries having the excellent cycle life are obtained whenthe DBP absorption of the powdery mixture is not greater than 150 ml/100g. The results show that the reliability of the battery is significantlyaffected by the lyophobic property of the powdery mixture included inthe gas absorbing element. Namely adjustment of the DBP absorption ofthe powdery mixture to be not greater than 150 ml/100 g keeps the gasabsorbent from being wetted with the non-aqueous solvent, thus ensuringlong-term, stable gas absorption capacity.

EXAMPLE 5

The powdery mixture prepared in Example 1 was molded under varyingpressures of 50 to 300 kgf/cm² to a plate-like shape of 15 mm in length,47 mm in width, and 0.3 mm in thickness, and was sintered in a flow ofnitrogen at 300° C. for 30 minutes. This gave multiple plate-like gasabsorbing elements. The surface free energy of the gas absorbingelements was in a range of 30 to 13 mN/m at 20° C. The variation insurface free energy is ascribed to the changed surface conditions of thegas absorbing element due to the varying molding pressure. Rectangularbatteries (Mc_(n)) were then manufactured in the same manner as Example2, except that the respective gas absorbing elements were used.

Evaluation of Batteries 3

The batteries Mc_(n) of Example 5 were subjected to the charge-dischargecycle test according to the procedure discussed in Evaluation ofBatteries 1. Table 2 shows the mapping of the difference between thesurface free energy of the gas absorbing element and the surface freeenergy of the non-aqueous electrolyte to the cycle life of the battery.

TABLE 2 Surface Free Energy Difference Cycle Life Example Batteries(mN/m) (times) Ex. 5 Mc₁ 10 216 Mc₂ 15 300 Mc₃ 20 360 Mc₄ 23 380 Mc₅ 27395

As clearly seen in Table 2, the greater difference in surface freeenergy gave the battery of the longer life. According to the results,regulating the difference between the surface free energy of the gasabsorbing element and the surface free energy of the non-aqueouselectrolyte (40 mN/m) keeps the gas absorbent from being wetted with thenon-aqueous solvent, thus ensuring long-term, stable capacity ofabsorbing the produced gas.

EXAMPLE 6

Gas absorbing elements were produced and rectangular batteries (Md_(n))were manufactured in the same manner as Example 2, except that the PTFEpowder as the lyophobic agent was replaced with polyethylene powder,polypropylene powder, polyvinylidene fluoride powder, polyacrylonitrilepowder, or SBR powder. Any of these lyophobic agents had the meanparticle diameter of 1.0 μm.

Evaluation of Batteries 4

The batteries Md_(n) of Example 6 were subjected to the charge-dischargecycle test according to the procedure discussed in Evaluation ofBatteries 1. Table 3 shows the mapping of the various lyophobic agentsto the difference between the surface free energy of the gas absorbingelement and the surface free energy of the non-aqueous electrolyte andthe life of the battery.

TABLE 3 Surface Free Cycle Energy Difference Life Example BatteriesLyophobic Agent (mN/m) (times) Ex. 6 Md₀ PTFE 25 380 Md₁ Polyethylene 10180 Md₂ Polypropylene 13 210 Md₃ Polyvinylidene 3 72 fluoride Md₄Polyacrylonitrile 6 168 Md₅ SBR 18 300

As clearly seen in Table 3, when the difference in surface free energyis not less than 5 mN/m, the resulting battery has the sufficiently longlife, regardless of the variation in lyophobic agent.

EXAMPLE 7

Gas absorbing elements were produced and rectangular batteries (Me_(n))were manufactured in the same manner as Example 2, except that carbonblack-type active carbon as the gas absorbent was replaced with carbonblack, zeolite, pitch-type active carbon, or palm shell active carbon.

Evaluation of Batteries 5

The batteries Men of Example 7 were subjected to the charge-dischargecycle test according to the procedure discussed in Evaluation ofBatteries 1. The battery using the pitch-type active carbon had thelongest life, and the life of the battery was shortened in the order ofcarbon black-type active carbon, palm shell active carbon, zeolite, andcarbon black. The results show that the pitch-type active carbon, carbonblack-type active carbon, and palm shell active carbon are especiallypreferable for the gas absorbent.

EXAMPLE 8

(i) Production of Gas Absorbing Element

100 parts by weight of the gas absorbent (carbon black-type activecarbon) prepared in Example 1 was mixed with 5 parts by weight ofsaccharose and an adequate quantity of water. The mixture was moldedunder a pressure of 150 kgf/cm² to a plate-like shape of 15 mm inlength, 47 mm in width, and 0.3 mm in thickness, and the molded articlewas sintered in a flow of nitrogen at 300° C. for 30 minutes.

The PTFE powder used in Example 1 was then homogeneously sprayed overthe whole face of the sintered article, and the article was furtherheated at 300° C. for 30 minutes to sinter the PTFE powder and form aporous layer of 20 μm in thickness. The surface free energy of thesintered article covered with the porous layer was 18 mN/m at 20° C.

(ii) Production of Rectangular Battery

A rectangular battery was manufactured in the same manner as Example 2,except that the plate-like sintered body covered with the porous layerwas used as the gas absorbing element. This battery is expressed as Mf.

COMPARATIVE EXAMPLE 4

A battery similar to Example 8 was manufactured, except that thesintered article of carbon black-type active carbon without the porouslayer was used. This battery is expressed as Mr₂.

Evaluation of Batteries 6

The batteries Mf and Mr₂ were subjected to the charge-discharge cycletest according to the procedure discussed in Evaluation of Batteries 1.The results are shown in Table 4.

TABLE 4 Surface Free Energy Gas Absorbing Difference Cycle Life ExampleBatteries Element (mN/m) (times) Ex. 8 Mf Molded article 22 375 coveredwith porous layer Com.Ex. 4 Mr₂ Molded article 3 55 Alone

As clearly seen in Table 4, the life of the battery Mf with thelyophobic agent-provided gas absorbing element is significantly longerthan the life of the battery Mr₂ with the gas absorbent alone.

EXAMPLE 9

The following describes application of the gas absorbing elementincluding the selective gas absorbent.

(i) Preparation of Selective Gas Absorbents

(a) Selective Gas Absorbent a

Palm shell active carbon obtained by firing the palm shell attemperatures of 400 to 650° C. was used as the powdery carbon material.The powdery carbon material was soaked in a 10N aqueous KOH solution,was dehydrated, and was exposed to steam having temperature of not lowerthan 110° C. for 24 hours for activation. The obtained carbon materialafter the above activation process (specific surface area: 120 m²/g; DBPabsorption: 110 ml/100 g) is expressed as r.

The carbon material r was then located in a flow of Ar and thetemperature was increased at a rate of 10° C./minute up to 600° C. toheat the carbon material r at the temperature for 1 hour. This gave aselective gas absorbent a having the capacity of selectively absorbingmethane and carbon dioxide.

(b) Selective Gas Absorbent b

Commercially available carbon black was used as the powdery carbonmaterial. This powdery carbon material was subjected to the sameactivation process as the carbon material r. The obtained carbonmaterial is expressed as s.

The carbon material s was then located in a flow of Ar and thetemperature was increased at a rate of 10° C./minute up to 600° C. toheat the carbon material s at the temperature for 1 hour. This gave aselective gas absorbent b having the capacity of selectively absorbingmethane and carbon dioxide.

(c) Selective Gas Absorbent c

The carbon material r was heated in a 1.5×10⁵ Pa gaseous mixtureatmosphere containing benzene and nitrogen at 600° C. for 2 hours. Thisgave a selective gas absorbent c having the capacity of selectivelyabsorbing carbon dioxide. The content of benzene in the gaseous mixturewas 5% by volume.

(ii) Evaluation of Selective Gas Absorbents and Carbon Material s

(a) Selectivity of Gas Absorption

In the procedure of the test, a fixed quantity of each of the selectivegas absorbents and the carbon material s was heated in a vacuumatmosphere at 400° C. for 2 hours, while degasifying the atmosphere to apressure of 1 Torr (1.3×10² Pa). Then, air at 25° C. was introduced intothe atmosphere to a pressure of 1 atm (1×10⁵ Pa). The sample was thenleft for one hour to confirm no variation in pressure of the atmosphere.A volume V1 of the air absorbed by each of the selective gas absorbentand the carbon material s was measured.

The procedure of the test then used the equi-volume gaseous mixture ofmethane and carbon dioxide, in place of the air at 25° C., and measureda volume V2 of the gaseous mixture absorbed by each of the selective gasabsorbents and the carbon material s. The ratio V2/V1 was thencalculated. The results are shown in Table 5.

The air volume V1 and the gaseous mixture volume V2 absorbed by 1 g ofthe selective gas absorbent a were respectively 0.12 ml and 8 ml.

(b) DBP Absorption

In the procedure of the test, each of the selective gas absorbents andthe carbon material s was heated in the vacuum atmosphere at 400° C. for2 hours, while degasifying the atmosphere to the pressure of 1 Torr.Then, each of the selective gas absorbents and the carbon material ssubjected to absorption of the air to the respective saturated levels ina 1×10⁵ Pa atmosphere of the air at 25° C. 100 g of each of theselective gas absorbents and the carbon material s was then soaked inDBP to make them sufficiently absorb DBP. After excess DBP was removed,the volume of DBP absorbed by each of the selective gas absorbents andthe carbon material s was measured. The results are shown in Table 5.

(c) Weight Decreasing Rate

In the procedure of the test, each of the selective gas absorbents andthe carbon material s was heated in the vacuum atmosphere at 400° C. for2 hours, while degasifying the atmosphere to the pressure of 1 Torr.Then, each of the selective gas absorbents and the carbon material s wassubjected to absorption of carbon dioxide to the respective saturatedlevels in a 1×10⁵ Pa atmosphere of carbon dioxide. The weight decreasingrates at 100° C. and at 500° C. of each of the selective gas absorbentsand the carbon material s was measured in a flow of Ar by differentialthermal analysis, and the ratio of the weight decreasing rate at 100° C.to the weight decreasing rate at 500° C. was calculated. The resultsexpressed as percent are shown in Table 5.

(d) O/C Ratio

The O/C ratio of each of the selective gas absorbents and the carbonmaterial s was measured by organic elemental analysis. The results areshown in Table 5.

(e) Specific Surface Area

The specific surface area of each of the selective gas absorbents andthe carbon material s was measured by the BET method. All of theselective gas absorbents and the carbon material s had the specificsurface area of greater than 500 m²/g.

TABLE 5 Ratio of Selective DBP Weight Gas Absorption DecreasingAbsorbents V2/V1 (ml/100 g) Rate (%) O/C Ratio a 65 96 5 0.12 b 175 1152 0.08 c 200 62 1 0.12 Carbon 0.2 459 50 0.2 Material s(iii) Production of Batteries(a) Battery 1 a

LiCoO₂ was used as the positive electrode active material. 100 parts byweight of LiCoO₂, 3 parts by weight of a conducting material, 10 partsby weight of polyvinylidene fluoride, and 70 parts by weight ofN-methyl-2-pyrrolidone were kneaded to yield a positive electrodematerial mixture. Polyvinylidene fluoride was dissolved inN-methyl-2-pyrrolidone. The positive electrode material mixture was thenapplied on both faces of an aluminum foil current collector having athickness of 20 μm, rolled, dried, and cut to a predetermined size. Thisgave a positive electrode plate having a thickness of 150 μm and aweight per unit area of 3.2 g/cm².

A 2:1 (weight ratio) mixture of the carbon material s and the selectivegas absorbent a was used for the conducting material. The content of theselective gas absorbent a in a resulting battery 1 a was 0.2 g.

Artificial graphite was used as the negative electrode active material.100 parts by weight of artificial graphite, 3 parts by weight of carbonblack as the conducting material, 8 parts by weight of polyvinylidenefluoride, and 70 parts by weight of N-methyl-2-pyrrolidone were kneadedto yield a negative electrode material mixture. Polyvinylidene fluoridewas dissolved in N-methyl-2-pyrrolidone. The negative electrode materialmixture was then applied on both faces of a copper foil currentcollector having a thickness of 15 μm, rolled, dried, and cut to apredetermined size. This gave a negative electrode plate having athickness of 160 μm and a weight per unit area of 1.3 g/cm².

The positive electrode plate and the negative electrode plate laid oneupon the other via a polyethylene film separator were wound in aflat-sided shape to give an electrode plate assembly, which wasaccommodated in a rectangular case. A 1:1 (volume ratio) solvent mixtureof ethylene carbonate and diethyl carbonate with LiPF₆ dissolved thereinat the concentration of 1 mol/liter was injected as the non-aqueouselectrolyte into the case. The opening of the case was closed with asealing plate and sealed by laser welding. A resulting rectangularbattery 1a had the length of 50 mm, the width of 34 mm, the thickness of6.3 mm, and the capacity of 850 mAh.

(b) Battery 1 b

The positive electrode and the negative electrode were the same as thoseof the battery 1 a. The positive electrode plate 1 and the negativeelectrode 2 laid one upon the other via a polyethylene film separatorwere wound in a spiral form to give a cylindrical electrode plateassembly, which was accommodated in a cylindrical case.

A 1:1 (volume ratio) solvent mixture of ethylene carbonate and ethylmethyl carbonate with LiPF₆ dissolved therein at the concentration of 1mol/liter was injected as the non-aqueous electrolyte into the case. Theopening of the case was sealed with a sealing plate. A resultingcylindrical battery 1 b had the diameter of 18 mm, the height of 65 mm,and the capacity of 1800 mAh. The content of the selective gas absorbenta in the battery 1 b was 0.4 g.

(c) Battery 1 c

The positive electrode material mixture was prepared in the same manneras the battery 1 a, except that polyvinylidene fluoride was replaced byPVDF-HFP. This positive electrode material mixture was applied on asingle face of an aluminum foil current collector having a thickness of20 μm, rolled, dried, and cut to a predetermined size. This gave apositive electrode plate 1 having a thickness of 150 μm and a weight perunit area of 2.3 g/cm². The negative electrode plate was the same asthat of the battery 1 a.

One negative electrode plate was interposed between two positiveelectrode plates, with the face of the positive electrode materialmixture inside, to assemble a stack of electrode plates. Separatorlayers having a thickness of about 25 μm and comprisingN-methyl-2-pyrrolidone with PVDF-HFP dissolved therein were locatedbetween the positive electrode material mixture and the negativeelectrode material mixture. The stack of electrode plates was insertedinto a bag-shaped case of a laminate sheet including a resin film and analuminum foil.

The 1:1 (volume ratio) solvent mixture of ethylene carbonate and ethylmethyl carbonate with LiPF₆ dissolved therein at the concentration of 1mol/liter was injected as the non-aqueous electrolyte into the case. ThePVDF-HFP in the separator layer was gelated. The opening of the case wassealed with a thermoplastic resin. A resulting polymer battery 1 c hadthe length of 70 mm, the width of 63 mm, the thickness of 3.6 mm, andthe capacity of 1070 mAh. The content of the selective gas absorbent ain the battery 1 c was 0.2 g.

(d) Batteries 2 a, 2 b, and 2 c

A rectangular battery 2 a, a cylindrical battery 2 b, and a polymerbattery 2 c were prepared respectively under the same conditions asthose of the batteries 1 a, 1 b, and 1 c, except that the selective gasabsorbent a was replaced by the selective gas absorbent b.

(e) Batteries 3 a, 3 b, and 3 c

A rectangular battery 3 a, a cylindrical battery 3 b, and a polymerbattery 3 c were prepared respectively under the same conditions asthose of the batteries 1 a, 1 b, and 1 c, except that the selective gasabsorbent a was replaced by the selective gas absorbent c.

(f) Batteries 4 a, 4 b, and 4 c of Comparative Examples

A rectangular battery 4 a, a cylindrical battery 4 b, and a polymerbattery 4 c were prepared respectively under the same conditions asthose of the batteries 1 a, 1 b, and 1 c except that the selective gasabsorbent a was replaced by the carbon material s.

(g) Batteries 5 a, 5 b, and 5 c of Comparative Examples

A rectangular battery 5 a, a cylindrical battery 5 b, and a polymerbattery 5 c were prepared respectively under the same conditions asthose of the batteries 1 a, 1 b, and 1 c, except that the selective gasabsorbent a and the carbon material s were replaced by the carbonmaterial r.

(iv) Evaluation of Batteries

Prior to evaluation, each battery was initially charged to a voltage of4.2 V with an electric current of 0.2 C (5-hour rate) on the basis ofthe theoretical capacity of the battery.

Evaluation 1

The batteries 1 a, 2 a, 3 a, 4 a, and 5 a were subjected to a cycle testat 45° C. The cycle test repeated a cycle, which discharged each batteryto 3.0 V with an electric current of 1 C (1-hour rate), charged thebattery to 4.25 V with an electric current of 0.7 C, and further chargedthe battery at a fixed voltage to attain an electric current level of0.05 C. The test then measured a variation in discharge capacity. Thepercent of the observed discharge capacity relative to the initialdischarge capacity in the first cycle as 100% was plotted against thenumber of cycles. The results are shown in FIG. 18.

In the graph of FIG. 18, the capacities of the batteries 4 a and 5 awithout the selective gas absorbent were significantly lowered with theprogress of the cycle. The capacities of the batteries 1 a through 3 ain Examples including the selective gas absorbent were, on the otherhand, kept at high levels.

After the evaluation test, the battery was decomposed for observation.Gas bubbles were present between the electrode plates in the batteries 4a and 5 a in Comparative Examples. The state of contact between theadjoining electrode plates was rather insufficient. In the batteries 1 athrough 3 a in Examples, on the contrary, practically no polarizationwas observed. This is ascribed to the function of the selective gasabsorbent, which absorbs carbon dioxide and methane in the batteries 1 athrough 3 a. The cylindrical batteries 1 b through 5 b, and the polymerbatteries 1 c through 5 c were also evaluated in the same manner.Similar tendency to that of the batteries 1 a through 5 a was observedin these batteries 1 b through 5 b and 1 c through 5 c.

Evaluation 2

The polymer batteries 1 c through 5 c were subjected to experiments 1through 3 discussed below, and the state of each battery after theexperiment was observed. In order to clarify the effects of the presentinvention, the safety valve was fully closed in the batteries 1 cthrough 5 c. The results are shown in Table 6.

-   Experiment 1: Each battery overcharged at 4.25 V was kept at 60° C.    for 20 days, and the increase in thickness of the battery (bulge)    was measured.-   Experiment 2: Each battery was subjected to a cycle test. The cycle    was repeated 20 cycles. The cycle discharged the battery to a    voltage of 3.0 V with an electric current of 1 C at 20° C., charged    the battery to 4.25 V with an electric current of 0.7 C, and further    charged the battery at a fixed voltage to attain an electric current    level of 0.05C. The increase in thickness of the battery was then    measured.-   Experiment 3: The same cycle test as the experiment 2 was performed,    except that the environmental temperature was 45° C. The increase in    thickness of the battery was measured after the cycle test.

TABLE 6 Batteries Experiment 1 Experiment 2 Experiment 3 1c 0.03 mm 0.10mm 0.30 mm 2c 0.01 mm 0.05 mm 0.20 mm 3c 0.01 mm 0.03 mm 0.15 mm 4c 0.70mm 1.10 mm 1.60 mm 5c 0.70 mm 1.15 mm 1.75 mm

As clearly shown in Table 6, the thickness was significantly increasedin the batteries 4 c and 5 c of Comparative Examples. This shows thatthe internal pressure in the batteries 4 c and 5 c drastically increasedafter the experiments 1 through 3. In the batteries 1 c through 3 c ofExamples, only a little increase in thickness was observed. The resultsclearly show that the selective gas absorbent in the battery effectivelyprevents an increase in internal pressure of the battery and therebydeformation of the battery.

As described above, the technique of the present invention utilizes thegas absorbing element that is not readily wetted with a non-aqueoussolvent and ensures long-term, stable action, thus remarkably enhancingthe reliability of the non-aqueous electrolyte secondary battery. Thetechnique of the present invention also utilizes the gas absorbent thatselectively absorbs methane and carbon dioxide produced inside thebattery, thus remarkably enhancing the reliability of the non-aqueouselectrolyte secondary battery. The principle of the present invention iseffective for any non-aqueous electrolyte secondary batteries containinga non-aqueous solvent and is thus applicable to small-sized batteriesused for small-sized electronic apparatuses, as well as large-sizedbatteries used for electronic vehicles and power reservoirs.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A non-aqueous electrolyte secondary battery, comprising: an electrodeplate assembly including a positive electrode, a negative electrode, anda separator interposed between said positive electrode and said negativeelectrode; a non-aqueous electrolyte including a lithium salt and anon-aqueous solvent; and a separate gas absorbing element capable ofabsorbing gas produced in said secondary battery, wherein said gasabsorbing element comprises a gas absorbent and a lyophobic agentagainst said non-aqueous solvent, and said gas absorbent comprises acarbon material with benzene chemically adsorbed thereon.
 2. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said gas absorbent is capable of absorbing at least one gasselected from the group consisting of methane, ethane, ethylene, carbondioxide, and hydrogen.
 3. The non-aqueous electrolyte secondary batteryin accordance with claim 1, wherein said lyophobic agent comprises atleast one selected from the group consisting of polyethylene,polypropylene, polytetrafluoroethylene, polyvinylidene fluoride,polyacrylonitrile, polyimide, copolymer of tetrafluoroethylene andhexafluoropropylene, and copolymer of styrene and butadiene.
 4. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein dibutyl phthalate absorption of said lyophobic agent is notgreater than 150 ml/100 g.
 5. The non-aqueous electrolyte secondarybattery in accordance with claim 1, wherein said gas absorbing elementcomprises a powdery mixture including said gas absorbent and saidlyophobic agent.
 6. The non-aqueous electrolyte secondary battery inaccordance with claim 5, wherein a content of said lyophobic agent insaid powdery mixture is 2 to 30 parts by weight per 100 parts by weightof said gas absorbent.
 7. The non-aqueous electrolyte secondary batteryin accordance with claim 5, wherein dibutyl phthalate absorption of saidpowdery mixture is not greater than 150 ml/100 g.
 8. The non-aqueouselectrolyte secondary battery in accordance with claim 5, wherein saidgas absorbing element is either one of a molded article and a sinteredarticle of said powdery mixture.
 9. The non-aqueous electrolytesecondary battery in accordance with claim 1, wherein a differencebetween surface free energy of said gas absorbing element and surfacefree energy of said non-aqueous electrolyte is 5 to 50 mN/m at 20° C.10. The non-aqueous electrolyte secondary battery in accordance withclaim 1, wherein said gas absorbing element has a porous layerpreventing said gas absorbent from being wetted excessively by saidnon-aqueous electrolyte, and said porous layer comprises said lyophobicagent.
 11. The non-aqueous electrolyte secondary battery in accordancewith claim 10, wherein a difference between surface free energy of saidporous layer and surface free energy of said non-aqueous electrolyte is5 to 50 mN/m at 20° C.
 12. The non-aqueous electrolyte secondary batteryin accordance with claim 1, wherein said gas absorbing element comprisesa coating film formed on a surface of a constituent of said battery,said coating film comprising said gas absorbent and said lyophobicagent.
 13. The non-aqueous electrolyte secondary battery in accordancewith claim 1, wherein said gas absorbing element comprises a coatingfilm formed on a surface of a constituent of said battery and a porouslayer covering said coating film, said coating film comprising said gasabsorbent, and said porous layer comprising said lyophobic agent. 14.The non-aqueous electrolyte secondary battery in accordance with claim1, wherein said electrode plate assembly is wound, and said gasabsorbing element is serving as a core of said wound electrode plateassembly.
 15. The non-aqueous electrolyte secondary battery inaccordance with claim 1, wherein said gas absorbing element is fixed toa sealing plate, said sealing plate sealing an opening of a casing foraccommodating said electrode plate assembly therein.
 16. A non-aqueouselectrolyte secondary battery, comprising: an electrode plate assemblyincluding a positive electrode, a negative electrode, and a separatorinterposed between said positive electrode and said negative electrode;a non-aqueous electrolyte including a lithium salt and a non-aqueoussolvent; and a separate gas absorbing element capable of absorbing gasproduced in said secondary battery, wherein said gas absorbing elementcomprises a gas absorbent capable of selectively absorbing at least oneselected from the group consisting of carbon dioxide and methane, andsaid gas absorbent comprises a carbon material with benzene chemicallyadsorbed thereon.
 17. The non-aqueous electrolyte secondary battery inaccordance with claim 16, wherein dibutyl phthalate absorption of saidgas absorbent is not greater than 150 ml/100 g.
 18. The non-aqueouselectrolyte secondary battery in accordance with claim 16, wherein saidgas absorbent has a specific surface area of 300 to 1500 m²/g, and aratio of number of oxygen atoms to number of carbon atoms: O/C ratio insaid gas absorbent, is not greater than 0.1.
 19. The non-aqueouselectrolyte secondary battery in accordance with claim 16, wherein saidgas absorbent after subjected to 2-hour degasification at 400° C iscapable of absorbing carbon dioxide, methane, or a gaseous mixture ofcarbon dioxide and methane by a volume of at least 10 times that of theair.
 20. The non-aqueous electrolyte secondary battery in accordancewith claim 16, wherein said gas absorbent after subjected to absorptionof the air to a saturated level in a 1×10⁵ Pa atmosphere of the air at25° C. is capable of absorbing carbon dioxide, methane, or a gaseousmixture of carbon dioxide and methane at a rate of at least 100 ml pergram in a 1×10⁵ Pa atmosphere of carbon dioxide, methane, or saidgaseous mixture of carbon dioxide and methane at 25° C.
 21. Thenon-aqueous electrolyte secondary battery in accordance with claim 16, aweight decreasing rate at 100° C. of said gas absorbent after subjectedto 2-hour degasification at 400° C. and subsequent absorption of carbondioxide to a saturate level in a 1×10⁵ Pa atmosphere of carbon dioxideat 25° C., in a flow of Ar by differential thermal analysis is notgreater than 10% of a weight decreasing rate at 500° C.
 22. Thenon-aqueous electrolyte secondary battery in accordance with claim 16,wherein a content of said gas absorbent in said battery is not less than0.2 g per 1000 mAh of cell capacity.
 23. The non-aqueous electrolytesecondary battery in accordance with claim 1, wherein said gas absorbentcomprises a carbon material obtained by heating a carbon powder in anbenzene atmosphere at 600 to 1000° C., so as to make benzene chemicallyadsorbed on said carbon powder.